In shearing lines, the oncoming sheets, especially thick plates, will be cut to a constant width by means of double trimming shears cutting along each side. Later the sheet can be split into two strips of more or less the same width by means of longitudinal shears. In order to accomplish this, the longitudinal shears can be shifted transversely of the sheet movement and can be locked firmly against the bed plate at each transverse adjustment point.
The longitudinal shears are equipped with two pairs of driving rollers at the inlet and outlet sides, one at each drive, one of which is secured in place, whereas the other can be made to move transversally to the movement of the sheet in correspondence to the different sheet widths. The separation between the inlet and outlet drivers is approximately 6 meters, the inlet driver introduces the front end of the sheet into the longitudinal shears and the shears start with the longitudinal cut of the sheet. As soon as the sheet front end reaches the outlet driver, the latter will close in such a way that the sheet will be driven by both drivers intermittently through the longitudinal shears and, while advancing, will be longitudinally cut. When the end of the sheet reaches the inlet driver, it opens, so that any further movement of the sheet will be accomplished only through the action of the outlet driver.
In the known longitudinal shears, not only the inlet driver but also the outlet driver is anchored to the base plate. The longitudinal shears are also movable transversally relative to the cut line over the base plate, so that it is adjustable independently of the drivers and can be driven over sheets of different widths.
In these knoown constructions of longitudinal shears there exists a rest force, during the longitudinal cut, which acts in addition to the shearing force and the chewing pressure. This rest force is transmitted, first from the sheet onto the driver aand then from the shears directly onto the base. This force is not exactly controlled and depends on the knife aperture, on the tensile strength of the sheet, and on the sheet thickness. Depending on the size, this force can produce a shifting of the longitudinal shears with respect to the sheet during the shearing effort, thus leading to inadmissible measurement errors on the finished sheets. Besides, there is also the disadvantage that, in cutting the sheets longitudinally, the cutting border at the beginning and end areas deviates from a straight line in an undesired manner; on the other hand, cutting border with good tolerance lies within the two end areas. It can be concluded that the deviations from the straight line occur on those areas where the sheet is guided by only one of the two drivers during the longitudinal cut. These inaccuracies stem from the fact that, in the case of sheets guided only from one side in the area of the longitudinal shears, the pressure exerted by the upper blade produces a shifting of the sheet in the direction out of the upper blade side, therefore, producing a separating cut which deviates from a straight line.
In order to avoid these unacceptable deviations, the longitudinal shears have been equipped with guide arms with vertically guided rollers which are located in the cutting area of the longitudinal shears, but lie before and after the latter. While working with cutting lines, however, when trimming double shears are located ahead of the longitudinal shears, the tolerances of the width of the sheet cannot be eliminated. The sheet has a characteristic elasticiity in its plane and still undesired lever-like conditions are presented. Thus, the addition of guiding arms has not conducted to any practical improvement.
Contrary to the above, a better working result was obtained in the end area of the sheet by the location of a pair of holding rollers in a separation of 3 to 4 meters approximately, behind the driver at the outlet side. A corresponding improvement of the working conditions should be obtainable through the installation of an equal holding roller pair before the driver at inlet side. It has been shown, however, that this kind of additional holding roller guiding does not give the same results for all sheet measurements. It has been found, on the contrary, that with thick and wide sheets the deviation of the cut from a straight line is in a sense satisfactory, whereas for thin, narrow sheets, the piece of sheet resulting from the longitudinal cut while being held by the driver and the holding roller pair, will be elastically deformed and impressed by the upper blade. In this case, in spite of the dual clamping, a deviation from a straight line is encountered.
The purpose of the invention is to eliminate these disadvantages. Thus, it is the purpose of the invention to produce a longitudinal shear, of the kind described at the beginning, such that, during the shearing work, a transverse shifting of the longitudinal shear cannot occur. In addition, accurate guiding of the sheet takes place independently of its dimensions or elasticity, so that at the beginning and end areas of the longitudinally-sheared sheet, a resulting straight lineal cut is assured.